The present invention generally relates to scrambler systems, and more particularly to a scrambler system which is suited for application in radio communication when transmitting an audio signal.
Recently, radio communication systems are applied to general telecommunication. Cordless telephone systems are already reduced to practice, and there is a first telephone system wherein a main telephone set (connecting apparatus of the cordless telephone) which is connected to a telephone line and a cordless sub telephone set (hand set of the cordless telephone) are connected by radio. There is also a second telephone system wherein a car telephone set is coupled to a telephone line by radio via a relay station. In the case of the first telephone system, the radio transmission and reception between the main telephone set and the sub telephone set can be performed within a range of 200 meters, for example, and for this reason, it is possible to intercept the radio communication by another sub telephone set or a high fidelity radio set within the 200 meter range. On the other hand, in the case of the second telephone system, the radio transmission and reception between the car telephone set and the relay station can be intercepted by a high fidelity radio set. Hence, there are scrambler systems for scrambling the audio signal before the radio transmission so that the content of the audio signal will be unintelligible to a person who makes the radio interception. According to the scrambler system, the audio signal is scrambled in accordance with a predetermined rule before the radio transmission. Hence, the content of the radio transmission will be unintelligible unless the transmitted signal is de-scrambled in accordance with the rule (key) which was used to scramble the audio signal before the radio transmission. In other words, the audio signal which is transmitted by use of such a scrambler system cannot be de-scrambled back into the original audio signal unless the key of the scrambler system is known, and the scrambler system is effective in preventing radio poaching by the radio interception.
As examples of the conventional scrambler system, there are scrambler systems which process the audio signal on the frequency axis before the radio transmission. According to a first scrambler system, a transmitter frequency-converts the audio signal before the radio transmission, and a receiver frequency-converts the received frequency converted audio signal back into the original audio signal. The circuit construction of the first scrambler system is simple, however, there is a disadvantage in that it is easy to see through the first scrambler system by guessing the frequency of the frequency converting signal. According to a second scrambler system, a transmitter divides the frequency band of the audio signal into a plurality of frequency bands and transmits signals in the divided frequency bands in a predetermined sequence, and a receiver restores the received signals in the divided frequency bands back into the original sequence so as to obtain the original audio signal. It is difficult to see through the second scrambler system, however, there is a disadvantage in that the circuit construction is complex.
On the other hand, as other examples of the conventional scrambler system, there are scrambler systems which process the audio signal on the time base before the radio transmission. According to a third scrambler system, a transmitter compresses and expands arbitrary parts of the audio signal on the time base before the radio transmission, and a receiver performs expansion and compression complementary to those performed in the transmitter so as to obtain the original audio signal. It is difficult to see through the third scrambler system, however, there are disadvantages in that the circuit construction is complex and the sound quality becomes deteriorated when the expansion and compression are performed in the receiver so as to obtain the original audio signal. According to a fourth scrambler system, a transmitter samples the audio signal at a predetermined sampling frequency and transmits the sampled data in a predetermined sequence, and a receiver restores the received sampled data back into the original sequence so as to obtain the original audio signal. It is difficult to see through the fourth scrambler system, but there is a disadvantage in that the circuit construction is complex. In addition, there is another disadvantage in that the sound quality becomes deteriorated when restoring the received sampled data back into the original sequence and obtaining the original audio signal.
As still another example of the conventional scrambler system, there is a fifth scrambler system which processes the audio signal digitally before the radio transmission. According to the fifth scrambler system, a transmitter converts the audio signal into a digital signal and interleaves the digital signal and transmits the interleaved digital signal, and a receiver de-interleaves the received interleaved digital signal so as to obtain the original audio signal. It is difficult to see through the fifth scrambler system, however, there are disadvantages in that the circuit construction is complex and the transmission must be performed digitally.
Hence, there is a demand for a scrambler system which has a large number of keys so that it is difficult to guess and see through the scrambler system, but has a simple circuit construction.